1. Field of the Invention
Embodiments of the invention relate to an apparatus and to a method for evaluating sensor measurement values intended to be used for predicting decompensation. In this context, the sensor measurement values refer to the output signals of sensors for physiological parameters.
2. Description of the Related Art
Patients suffering from cardiac insufficiency can, at irregular intervals, experience an acute deterioration of the state of health, associated with a complete breakdown of the body's performance, shortness of breath, and fear of suffocation. In this case, immediate hospitalization with emergency care is required.
These events are caused by what is referred to as “decompensation”, which is an inability of the heart to maintain adequate blood circulation. This event manifests itself several weeks in advance by an (automatically intensifying) increase in pressure in the pulmonary circulation, and associated therewith by an increased accumulation of fluid in the lung tissue, which, however, is generally not perceived in time by the patient.
The development of decompensation, associated with tremendous stress for the patient and the cardiovascular system, considerably worsens the clinical picture of cardiac insufficiency. Early detection of developing decompensation can significantly increase the life expectancy of the patient. The potential prevention of hospitalization required due to a critical condition of the patient can lower the costs of medical care.
This requires a device which generates a warning in due time before the acute phase when decompensation is developing. As a result, corresponding therapeutic measures can be initiated at an early stage and hospitalization can be prevented.
Patients suffering from heart failure are increasingly supplied with implanted defibrillators (IDC), since part of this patient group is also at increased risk for life-threatening tachyarrhythmia.
The ICDs used are designed as single-chamber or dual-chamber systems. Approximately two thirds of all patients having heart failure suffer from systolic heart failure, in which the efficiency of blood ejection is severely worsened (for example, caused by a left bundle branch block). Today, such patients are increasingly treated with an implant, which through the synchronous stimulation of the right and left ventricles restores the synchronicity of the contractions of the left and right ventricles. The patients are given what is referred to as cardiac resynchronization therapy (CRT), wherein a pacemaker or ICD is provided with separate electrodes to the right and left ventricles (by way of the coronary sinus).
A physical parameter which allows the increasing accumulation of fluid in the thorax to be determined is the transthoracic electrical impedance. If the fluid content in the lung tissue increases, the impedance measured decreases. By using an implant and the implanted electrodes, this transthoracic impedance is easy to measure. In this way, decompensation can be detected at an early stage, prior to the acute phase, and the patient or physician can be issued a warning so as to initiate therapeutic measures.
Apparatuses measuring the transthoracic impedance between the implant housing and/or one or more cardiac electrodes in order to detect fluid accumulation in the lungs are known (U.S. Pat. No. 5,957,861, U.S. Pat. No. 6,076,015, U.S. Pat. No. 6,454,719, US 2006/0041280, US 2006/0258952, US 2006/0264776). In general, the impedance values are averaged over an extended period of time (such as over 24 hours) in order to compensate for the impedance variations caused by the cardiac and respiratory cycles and by circadian fluctuations. These averaged values serve as a basis for the early detection of developing pulmonary edemas; refer to Yu C M, Wang L, Chau E, Chan R H, Kong S L, Tang M O, Christensen J, Stadler R W, Lau C P. “Intrathoracic impedance monitoring in patients with heart failure: correlation with fluid status and feasibility of early warning preceding hospitalization.” Circulation 2005; 112 (6):841-8.
Secondary effects, which are not the result of the development of pulmonary edema, can likewise strongly influence the transthoracic impedance and simulate a fluid build-up, or hide it. These interferences must be compensated for with respect to the influence thereof on the detection of developing pulmonary edema.
One of these interferences can be caused by fluctuations in the blood resistivity, which may be due to a changed hematocrit level or the varying electrolyte content in the blood, for example.
In order to reduce the influence of blood resistivity, systems are described which determine the blood resistivity and use it to correct the transthoracic impedance (US 2006/0041280, US 2006/0258952, US 2006/0264776).
The impedances of pulmonary tissue and of blood exhibit different frequency characteristics. A system has been described which, for this reason, carries out the impedance measurement at different frequencies in order to minimize the influence of the blood resistivity (EP 1 665 983).
In addition, changes in the body position (such as getting up or lying down) result in a redistribution of the fluid in the body and therefore in a temporary change in the fluid content of the lungs. These changes in the fluid content are reflected in the transthoracic impedance due to the functional principle thereof and can potentially be interpreted as developing pulmonary edema. For this reason, systems have been described which detect the body position, or the change thereof, and include it in the assessment of the transthoracic impedance (US 2006/0041280, US 2006/0258952, US 2006/0264776). Additional interference can be created by fluid accumulations in the implant pocket or due to migration of the implant.
All these solutions have the disadvantage that additional sensors or additional measurements are required. The energy required for this can significantly reduce the service life, particularly in the case of long-term implants.
Also disclosed were systems which monitor respiration directly, so as to deduce a deterioration of the health condition (U.S. Pat. No. 5,876,353, U.S. Pat. No. 5,957,861, U.S. Pat. No. 6,076,015, U.S. Pat. No. 6,449,509, U.S. Pat. No. 6,454,719, US 2006/0258952) based on the respiratory rate or the respiratory rhythm that has been determined (such as shortness of breath, Cheyne-Stokes respiration, sleep apnea).
Respiration, however, is influenced only at a relatively late stage of the pulmonary edema; refer to Zipes, D. P. et al. [ed.]: Braunwald's Heart Disease; Elsevier, 2005. In addition, respiration is also influenced by a variety of other factors, such as physical strain, speaking and the general physical condition (NYHA). As a result, unequivocal early detection of pulmonary edema purely from respiration is very prone to errors. For example, the dependency of the impedance on respiration is also utilized in order to determine the respiratory minute volume, which is used to estimate the metabolic need and employed to control a frequency-adaptive pacemaker (U.S. Pat. No. 6,076,015, U.S. Pat. No. 6,449,509).
Furthermore, methods have been described which determine hemodynamic variables by way of an intracardiac impedance measurement value and from the change thereof derive a change in the condition. For this purpose, the dynamics of the heart beat is determined and fluctuations, which are caused by respiration and other influences, are eliminated, such as by averaging (Zima, E., et al. “Determination of left ventricular volume changes by intracardiac conductance using a biventricular electrode configuration.” Europace 8.7 (2006): 537-44; Stahl, C., et al. “Intracardiac Impedance Monitors Hemodynamic Deterioration in a Chronic Heart Failure Pig Model.” J. Cardiovasc. Electrophysiol. 18 (2007): 985-90; EP 1 510 173).
Also known are systems that combine several different parameters in order to better assess the progression of the disease and compensate for uncertainties of individual parameters (U.S. Pat. No. 5,876,353, U.S. Pat. No. 5,957,861, US 2006/0258952, US 2006/0264776).
The known solutions have a variety of disadvantages.
A crucial disadvantage of the known solutions is due to the fact that the transthoracic electrical impedance is influenced not only by the increasing fluid build-up in the lungs, but also by many other factors. Due to the influence of these secondary factors, the sensitivity and specificity of lung fluid detection are reduced. As a result, the secondary influencing factors must be compensated for by additional parameters or additional measured variables, at times also by different sensors (such as body position, intracardiac pressure sensors). Additional measurements increase the complexity and the consumption of energy. The increased energy consumption reduces the service life of the system, particularly in the case of long-term implants. The known respiration parameters are also dependent on many other factors and have little specificity taken by themselves. Again, the use of additional parameters is required.